1. Field of the Invention
The present invention relates to propellers and more specifically to an aeroelastic unitary laminated composite propeller.
2. Prior Art
Any propeller has a single most efficient pitch or angle of attack on the fluid through which it moves. An ideal perfectly rigid fixed-pitch propeller would be optimally pitched at only one speed and loading. Although actual pitch remains constant, the effective pitch of a propeller varies according to its forward and rotational speed relative to the fluid. The lower the forward speed the greater the effective angle of attack at a given rotational speed. The effective angle of attack and hence the resistance to rotation through a fluid is greatest at take-off or launching, when fluid resistance tends to prevent a typical engine from attaining the high rotational speed at which it produces the necessary maximum power. At cruising speed the effective angle is reduced and a greater actual angle is acceptable.
Variable pitch propellers are a well-known technique for changing the actual angle and thus improving the effective pitch, and offer the advantage that they can be designed to operate at a constant rotational speed, thus allowing the engine to operate at its single most efficient speed while producing different amounts of power at different forward velocities. However, variable pitch propellers have always been constructed from several pieces including a hub, bearings and articulated blades. Propellers using mechanisms to adjust pitch generally weigh twice as much as unitary propellers, need considerable maintenance, are failure prone, and are expensive. The propeller pitch can only be varied at the joint between the hub and each blade, and each blade is rotated as a rigid unit. Such variable pitch propellers have a narrow optimal range of operating conditions and are less efficient outside of that range. The range is much narrower for conventional unitary propellers.
Previous "aeroelastic" propellers have been made from homogeneous metal, wood or composite materials in attempts using various means to make the propellers deform advantageously to maintain an efficient shape. The increased centrifugal acceleration load on the blade at take-off can be employed to twist the blade so that its back side rotates forward around the blade's center of pressure line, which is approximately parallel to and about one-fourth of the blade width back from the leading edge. Thus, the pitch is lessened. However, homogeneous aeroelastic propellers can only respond to stresses in a limited number of ways, sometimes by compromising other desirable characteristics, and are not capable of differential deflection.
Airplane propellers deform due to stresses of inertial loads including centrifugal force, and aerodynamic loads, including lift, drag, and pitching moment. Propeller pitch also varies dynamically along the length of the blades according to acceleration-induced loads. Air pressure is greater on the back side of the blade, due to the airfoil cross-section shape and to the pitch of the blade. There is a bending load which integrates towards the hub. Although the accumulating stress near the hub can be offset by a thicker cross-section, the bending load still results in the blade tips pulling ahead in the direction of flight, or "coning."
As the propeller rotates it is subjected to centrifugal force which increases away from the hub and straightens radial curves in the blades. Thus, both "coning" and twisting are somewhat offset.
For all points on the rotating blades to screw the same distance through the fluid, the tips, traveling through a longer arc, need a smaller angle of attack than the "root" of the blade nearer the hub. The ratio of forward/tangential speed is highest and forward speed is a greater factor in the effective angle near the hub. To track the optimum effective angle of attack at different forward speeds, the blade would have to change pitch by greater amounts nearer the hub. This problem is addressed neither by the conventional scimitar blade, nor by any other presently known method of changing blade twist along the length of the blade. Viewed from the front, the "scimitar" blades are swept rotationally aft with increasing radius. The central portion of the blade, having a longer moment arm from the primary axis of the blade, twists further and the tips further yet. Scimitar blades are dangerous because centrifugal force tending to straighten the blade exerts extreme tension on the trailing edge, which is the thinnest part of the blade. Failures become more likely above about 1300 rpm, while contemporary airplane engines operate in the range of 2000 to 4000 rpm.
Other attempts to use loads to controllably deform propellers have employed wood laminates. The Warnke Company of Tuscon, Ariz., has used plywood to construct one-piece aeroelastic propellers with straight blades which are curved back around the axis of rotation to counter the coning effect. Such propellers are reported to have exhibited some of the desired deformations under load. However, the wooden propeller must be hand carved and because of its relatively low strength cannot employ high efficiency thin airfoil sections. Like all naturally occurring materials, wood is inconsistent, unpredictable and always varies from piece to piece. Successes have been sporadic and a more consistent, reliable and higher strength structure is needed. Wood is not as strong as other propeller materials, and therefore wooden blades must be made thicker and less efficient than metal or advanced composite blades.
Various uniaxial fibers, for example glass, boron, carbon, or Dupont Kevlar.RTM., may be embedded in a stabilizing agent, such as epoxy resin, to form substantially unidirectional plies. The plies can be cross-laminated to produce a planar material with correspondingly different strengths along its axes depending on the fiber orientations. Fibers may be chosen with a lower elastic modulus than metals used in propeller construction, which allows them to bend more before breaking than metals of equal breaking strength. These lower elastic modulus materials also possess better vibration damping, impact and shatter resistance, fatigue strength, and generally higher reliability which, with their very high strength-to-weight ratios, make these materials highly desirable for aircraft construction. Fibers may also be chosen with higher modulus than conventional metal materials, and blends of various fibers may be selected to achieve a variety of specific mechanical properties, depending upon the structural performance desired in the final laminate. Such a fiber ply will bend in a predictable and reliable manner either parallel or at an angle to the fiber axes, and it has been speculated that this might be a useful characteristic in designing an aeroelastic propeller. Theoretical studies on advanced fiber laminate propellers are presented, for example, in a paper entitled "Aeroelastically Tailored Propellers" by Dwyer and Rogers at a meeting of the Society of Automotive Engineers, in April 1977 and in another paper entitled "Development of an Aircraft Composite Propeller" by Harlamert and Edinger at a Society of Automotive Engineers meeting in April 1979. Engineering techniques for calculating the bending strength of a laminate of plies at given orientations are explained in Introduction to Composites by Hahn and Tsai (Technomic Publishing Co., Westport, Conn.). However, the problem of orienting plies in a propeller for optimum non-uniform deflection has not previously been addressed and a practical and efficient aeroelastic laminated propeller taking advantage of these speculations has not previously been disclosed.